This application is submitted in response to RFA Hl-00-012: Ancillary Studies in Lung Disease Trials. The parent trial to which this application is linked, (U01-HL62514), is testing the hypothesis that low dose inhaled nitric oxide, administered to infants 500 to 1250 g birth weight, will produce an increase in survival without chronic lung disease (CLD) from 50 percent to 61 percent at 36 week post menstrual age. The parent study is a blinded, placebo controlled, three week trial of decreasing dose nitric oxide or placebo beginning between 7 and 21 days of age. Part of the rationale supporting the parent trial is that nitric oxide may moderate pulmonary inflammation, a crucial precursor of full-blown CLD. Although the parent trial includes measurements of tracheal aspirate interleukin 1B and interleukin-8, it cannot investigate mechanisms of, nor initiation, propagation, or persistence of, pulmonary inflammation. Our proposed study will examine the role of two members of the family of transmembrane receptors, Toll-like receptors (TLRs), found on leukocytes and other cells and upregulated in response to endotoxin and to other stimuli. These substances transduce the signal propagating inflammatory mediator production. We will quantitate protein expression and mRNA expression of TLR-2, and TLR-4 from leukocytes obtained from serial tracheal aspirate samples. We will perform these serial studies in a subset of enrolled patients who will mirror the parent clinical trial sample in terms of ethnicity, gender, and severity of underlying disease. Our specific aims are to determine if upregulation of TLR-2 and/or TLR-4 antedates development of CLD of prematurity and to determine if there is a significant correlation with severity of CLD. We will determine if the mechanism of improved outcome with NO administration occurs in association with NO-associated suppression of upregulation of TLR, a possibility for which we have supportive pilot data. Even if the parent study cannot reject the null hypothesis that nitric oxide will have no benefit in this disease, this mechanistic study will provide important new information about natural history of CLD and about crucial mechanisms of early pulmonary inflammation. Its results may also open up intriguing pathways for treatments aimed at selective diminution of pulmonary inflammation.